b. Root pressure is generated by means of active transport of ions and accumulation of them in the root. That in turn decrease the osmotic potential (and so the water potential) in the root cells that drives water from the soil to the root. As the root is gaining water by means of osmosis it produces positive hydrostatic pressure. Eventually water moves to the xylem vessels behind ions that are passively transported from the xylem parenchyma to the xylem through the inwardly rectifying channels. New portions of water entering xylem vessels push it higher and higer. Such situation occurs when no leaves are present or when the stomata are closed and when roots are appropriately aerated, since active transport of ions inward the root requires proper respiration.

When leaves are present, root pressure plays hardly any role in water translocation i plant (when stomata are closed).
When stomata are open, steam escapes from extracellular and substomatal cavities through the stomatal pore producing negative pressure that sucks up water from xylem and in turn from roots. As roots lose water they absorb new portion from the soil causing decrease of it in the rhizosphere. That drives the bulk flow of water towards the roots and provide permanent (if water is present in the soil of course) delivery of liquid.

Whoa, I'm tired. I'll tell something about translocation in phloem next time, since it's even more complex (but fascinating).